A fuel cell is an electrochemical device wherein hydrogen or an organic fuel is reacted with oxygen to generate electricity and to produce water. The reactants fed into a fuel cell need not be pure, for the oxygen can be taken from the is atmosphere and the hydrogen extracted from natural or synthetic fuels. The basic fuel process is efficient and pollution-free. However, since a single fuel cell yields an output of less than one volt, in order to provide a useful output it is necessary to connect a group of cells in series in a stack or in a flat formation, to create a power pack.
As a converter of energy, a fuel cell differs from a conventional voltaic cell or battery in which anode and cathode electrodes cooperate with an electrolyte, in that in a fuel cell the electrodes are invariable and catalytically active. Current is generated in a fuel cell by reactions on the electrode surfaces in contact with the electrolyte. These reactions involve (i) oxidation of hydrogen or organic fuel on an anode functioning as the negative electrode, as in a galvanic cell, giving rise to hydrated protons with the release of electrons, and (ii) reduction of oxygen on the cathode and producing water as electrons are consumed.
The first practical fuel cell was developed by General Electric (GE) for NASA in connection with the Gemini program. Included in this cell was an electrolyte in the form of a solid polymeric membrane, referred to as an “ionic-exchange membrane”. The GE membrane was composed of a lace-like organic structure with an ionic group bonded firmly thereto, and hydrogen ions loosely held in the polymer chain to provide sufficient mobility for ionic support.
Fuel cells having polymeric membranes are disclosed in U.S. Pat. Nos. 5,599,638 and 5,777,162. Commonly used, as a membrane material having proton-exchange properties is Nafion™, manufactured by DuPont. When methanol serves as the fuel, electro-oxidation of methanol takes place at the anode, while electro-reduction of oxygen then occurs at the cathode to yield water. Protons generated at the anode are transported directly across the membrane to the cathode. The flow of current is sustained by ions flowing through the cell and by electrons passing through an external load.
A solid proton-conductive membrane (PCM) of the type included in a fuel cell in accordance with the invention, is disclosed in PCT Publication WO 99/44245 in the name of the same Applicant. Used in this membrane are polymeric binders selected from a group that includes polyvinylidenedifluoride (PVDF) polytetrafluoroethylene and polymethylmethacrylate. Inorganic powders for preparing this membrane include SiO2, ZrO2, TiO2, Al2O3 and B2O3.
Also of prior art interest is the fuel cell disclosed in PCT publication WO 01/54216, in the name of the same Applicant. This cell includes an anode chamber to which fuel is supplied, and a cathode chamber to which oxygen is supplied. Interposed between the chambers is a solid electrolyte in the form of a proton-conducting membrane having fine pores whose diameter is smaller than 30 nm.
In hydrogen and in direct oxidation fuel cells (DOFC) liquid water tends to leak through the cathode. This water may include fuel, resulting in a loss of fuel as well as water. Liquid water leakage through the cathode of a fuel cell having a solid electrolyte membrane such as Nafion™ can be serious, since the osmotic drag of water is about three water molecules per proton, or eighteen water molecules for each methanol molecule. This (if all the water exits the cathode) gives rise to a loss of up to 10 g of water per one gram of methanol consumed in the cell reaction. And the osmotic drag of water by the protons may cause a build-up of hydraulic pressure at the catalyst layer of this cathode. This acts to increase leakage of liquid water or aqueous fuel solution through holes or cracks in the gas diffusion layer (GDL) of the cathode, as well as through the edge or periphery of the membrane and through the gap between the cathode-solid membrane-anode assembly (also termed in the art “membrane electrode assembly” or MEA) and the circular gasket of the cell.